In the commercial production of rubber modified resins of the type wherein rubber particles of alkadiene rubber grafted with a styrene-type monomer and acrylonitrile-type monomer are generally uniformly dispersed in a matrix of a copolymer of styrene-type monomer and acrylonitrile-type monomer, which resins are commonly called ABS resins, it has heretofore been a common commercial practice to use either suspension or emulsion polymerization procedures for the preparation thereof. A given product resin is produced either as a result of a single batch polymerization or as a result of several batch polymerization followed by a physical blending procedure involving mixture of preformed graft copolymers with other graft copolymers or with ungrafted copolymer. Because of the inherent cost, complexity and sensitivity associated with such manufacturing technology, the art has recently been attempting to develop different techniques which would permit one to manufacture such resins using cheaper, less complex, highly reliable procedures and equipment. A particularly promising different technique would involve the use of mass polymerization which avoids such problems as product separation and recovery from the water used for suspension or emulsion polymerization.
To prepare ABS resins by all-mass polymerization technology has been heretofore proposed, but the practical realization of such a manufacturing route on a commercial scale is full of problems because of the difficulties of producing a product resin which is cheap enough to be competitive and still has the necessary and desirable physical properties for molding and extruding and for formed, manufactured product applications. Because of these difficulties, the art has combined some of the mass polymerization technology with some of the emulsion and/or suspension technology in an overall process.
U.S. Pat. No. 3,950,455 and U.S. Pat. No. 3,957,912 have disclosed such hybrid processes wherein the rubbers are grafted in aqueous emulsion as grafted crosslinked rubber particles and extracted from the aqueous phase by monomers into a monomer phase having said grafted rubber particles. The monomer phase is then separated from the aqueous phase and mass polymerized to an ABS resin.
Various methods have been used to control the color of such resins such as antioxidants and the like or closer control of the styrene and acrylonitrile monomer compositions during polymerization to control compositional drift leading to color chromophones in the resin. Such methods have proven costly and difficult to control.
It has now been discovered unexpectedly that chromophoric development during the mass polymerization of monovinylidene aromatic monomers and alkenenitrile monomers having a dispersed alkadiene rubber grafted with said monomers dispersed therein can be inhitied by carrying the mass polymerization in the presence of sufficient carbon dioxide to provide an ABS resin having a substantially white color.